Analog multiplexing circuits

ABSTRACT

A multiplexer circuit includes a plurality of switched differential amplifier circuits, one of which can be selected at a time. Each switched differential amplifier includes a pair of differential inputs and a pair of differential outputs, with each pair of differential inputs accepting a corresponding pair of input signals. Each of the switched differential amplifier circuits is configured to present a current mode version of its input signals at its differential outputs when the switched differential amplifier circuit is selected, and to present substantially zero level output signals at its differential outputs when the switched differential amplifier circuit is deselected. The multiplexer circuit also includes a selector that accepts a select signal and selects one of the plurality of switched differential amplifier circuits based on said select signal. A current mirror is used to combine a pair of multiplexer outputs into a single ended output, a version of which is used for feedback.

PRIORITY CLAIM

This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/591,993, which was filed Jul. 29, 2004.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the field of integrated circuits, and more specifically to analog multiplexing circuits.

BACKGROUND

The purpose of analog multiplexing circuits is to select one input from a number of analog inputs and reproduce the selected input faithfully at an output. Ideally, the multiplexer is wideband, has no feed through of unselected inputs, and can handle a reasonably wide range of input voltages from the unselected channels without causing damage to the internal circuitry of the multiplexer.

Unfortunately, high-speed semiconductor processes do not support large voltages without breakdown. Unless a schottky diode is available, the only high-speed junction available for switching function is a transistor's base-emitter (or equivalent) junction, which can be limited to as little as 1.5V of reverse bias, severely limiting the magnitude of input signals. Unfortunately, schottky diodes can not be produced using many types of semiconductor processes. Accordingly, it would be beneficial to provide an analog multiplexing circuit that can handle large input voltages and achieve high (e.g., GHz) frequency responses, without requiring a schottky diode.

SUMMARY OF THE PRESENT INVENTION

Embodiments of the present invention are directed to analog multiplexing circuits, as well as the circuits that can be used to make up analog multiplexing circuits.

In accordance with an embodiment of the present invention, a multiplexer circuit includes a plurality of switched differential amplifier circuits, one of which can be selected at a time. Each switched differential amplifier circuits includes a pair of differential inputs and a pair of differential outputs, with each pair of differential inputs accepting a corresponding pair of voltage input signals including a first input signal and a second input signal. Each switched differential amplifier circuit also includes a pair of switched input stages, each of which accepts one of the first and second input signals.

In accordance with an embodiment of the present invention, each switched input stage is configured to, when selected, present a substantially unmodified version of its input signal at its output. Additionally, each switched input stage is configured to, when deselected, produce a rejection voltage signal of substantially equal magnitude and opposite polarity to its input signal in order to reject the input signal and thereby present a substantially constant voltage at its output regardless of variations in the input signal. In accordance an embodiment of the present invention, this rejection voltage signal is produced using a transconductance circuit. More specifically, the transconductance circuit converts the input signal to a proportional current signal that, when the switched input stage is selected, flows through a passive resistance in order to produce the rejection voltage signal.

Further features, embodiments and details, and the aspects, and advantages of the present invention will become more apparent from the detailed description set forth below, the drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are circuit diagrams of switched input stages, according to embodiments of the present invention.

FIG. 2 is a circuit diagram of a switched differential amplifier, according to an embodiment of the present invention.

FIG. 3 is a circuit diagram of a multiplexer, according to an embodiment of the present invention.

FIG. 4 is a circuit diagram of a multiplexing amplifier, according to an embodiment of the present invention.

FIG. 5 illustrates how embodiments of the present invention can be used to produce a triple 2:1 video multiplexer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1A is a circuit diagram of an input stage 102 according to an embodiment of the present invention. The input stage 102 is shown as including an input buffer A1, a resistor R2, a capacitor C2, a pair of transconductors TC1 and TC2, and a pair of switches SW1 and SW2. While use of the buffer A1 is customary to unload the input signal source, it is not strictly necessary, and thus need not be included. If the buffer A1 is used, an input signal Vin that is presented at an input to the buffer A1 is passed through the buffer A1, which presents a buffered version of the input signal Vin to resistor R2 and transconductors TC1 and TC2. If the buffer A1 is not used, the input signal Vin will be presented directly to resistor R2 and transconductors TC1 and TC2. As will be understood from the discussion of FIG. 1C below, in an accordance with an embodiment of the present invention, each trandconductor can be implemented using an RC circuit and a current mirror. Further, transconductors TC1 and TC2 may be referred to hereafter collectively, or individually, as a transconductance circuit.

When the switches SW1 and SW2 are in the selected positions (to the ground and supply positions, respectively, as shown in FIG. 1A), the buffered version of the input signal Vin simply passes through resistor R2 substantially unmodified and is presented at the output of the input stage 102 as Vout (capacitor C2 is used to pass the input signal Vin around resistor R2 at high frequencies). Thus, if the desire is to pass the input signal Vin through the input stage 102, the switches SW1 and SW2 should be connected as shown, such that the output currents of transconductors TC1 and TC2 are shunted away from resistor R2. The input stage 102 will be said to be “selected” when it produces a substantially unmodified voltage mode version of its input at its output, as just described.

To reject the input signal Vin, switches SW1 and SW2 are switched to the deselected positions (the positions opposite to those shown in FIG. 1A). When switches SW1 and SW2 are in the deselected position, a current sourced from transductor TC1 flows through resistor R2 and is sunk into the output of transconductor TC2. This transconductor current produces a voltage drop, which is opposite in phase to Vin, across resistor R2, pulling the Vout node of input stage 102 to a known “deselect” voltage that is set by the current and gm (i.e., transconductance) of the transconductors TC1 and TC2. This “deselect” voltage can be any value, but is nominally equal to the lowest voltage value of the multiplexer's intended common mode range. This “deselect” voltage is set by selecting the gm of each transconductor. This is done in practice by changing the current available (nominally, the current and gm of TC1 and TC2 are equal) from each transconductor. Since the transconductors are driven by the input signal Vin of the input stage, their resultant output currents track the input voltage, producing a rejection voltage which is equal, but opposite in phase, to the input voltage, Vin, thus cancelling the effects of Vin at the Vout node. The input stage 102 will be said to be “deselected” when it rejects its input signal, as just described.

FIG. 1B is a circuit diagram of an input stage 102′, according to another embodiment of the present invention. In this embodiment, the amplifier A1 acts as one of the transconductors, and thus, is labeled A1/TC1 (eliminating the need for the separate transconductor TC1 shown in FIG. 1A). While the buffer/transconductor1 A1/TC1 can be used to unload the input signal source, it is not strictly necessary, and thus need not be included. If buffer/transconductor1 (A1/TC1) is not physically included, then the input signal source (not shown) will function as the transconductor1 (TC1). As will be discussed below with reference to FIG. 1C, the trandconductor2 (TC2) can be implemented using an RC circuit and a current mirror.

FIG. IC shows an input stage 102″ including an input buffer/transconductor1 (A1/TC1), resistors R1 and R2, capacitors C1 and C2, a current mirror/transconductor2 (CM/TC2) and a switch SW1. While the buffer/transconductor1 (A1/TC1) can be used to unload the input signal source, it is not strictly necessary, and thus need not be included. If buffer/transconductor1 (A1/TC1) is not physically included, the input signal source (not shown) then functions as the first transconductor1 (TC1). If the buffer/transconductor1 (A1/TC1) is used, an input signal Vin that is presented at an input to the buffer/transconductor1 (A1/TC1) is passed through the buffer/transconductor1 (A1/TC1), which presents a buffered version of the input signal Vin to resistor R1 and resistor R2. If the buffer/transconductor1 (A1/TC1) is not used, the input signal Vin will be presented directly to resistors R1 and R2.

When the switch SW1 is in the selected position (the left position shown in FIG. 1C), the buffered version of the input signal Vin simply passes through resistor R2 substantially unmodified and is presented at the output of the input stage 102 as Vout (capacitor C2 is used to pass the input signal Vin around resistor R2 at high frequencies). Thus, if the desire is to pass the input signal Vin through the input stage 102, the switch SW1 should be connected as shown, such that the output current of the current mirror/transconductor2 (CM/TC2) is not passed to the output side of resistor R2. The input stage 102 will be said to be “selected” when it produces a substantially unmodified voltage mode version of its input at its output, as just described.

To reject the input signal Vin, the switch S1 is switched to the deselected position (the right position shown in FIG. 1A). When the switch SW1 is in the deselected position, the signal source (or, if included, the buffer/transconductor1 (A1/TC1)), the resistor R1, and the input of CM act as a transconductor, converting Vin to a control current. This control current, which is presented to the input of the current mirror/transconductor2 (CM/TC2), is a function of the capacity of the input signal source (or, if included, buffer/transconductor1 (A1/TC1)), the voltage drop across resistor R1, the value of resistor R1, and the input drop of the CM/TC2 input. If transconductor/current mirror CM/TC2 is a simple current mirror, and is designed such that its output current is substantially equal to its input control current, then current sourced from the input signal source (or, if included, buffer/transconductor1 (A1/TC1)) flows through resistor R2 and is sunk by the output of the transconductor/current mirror output (CM/TC2). The input signal source (or if included, buffer/transconductor1 (A1/TC1)), in this case, should have the capacity to provide at least twice the current sunk by the output of transconductor/current mirror CM/TC2. Assuming A1/TC1 capacity and also if R1=R2, then the current (I) flowing from R2 at the Vout node, will cause a voltage drop across R2 in direct proportion, but opposite in phase, to that of the driving input signal, Vin, thus creating a constant voltage substantially equal to V1 at the Vout node, canceling the Vin signal. The input stage 102 will be said to be “deselected” when it rejects its input signal, as just described.

FIG. 2 is a circuit diagram of a switched differential amplifier 202 according to an embodiment of the present invention. As shown, the switched differential amplifier 202 includes a pair of switched input stages 102 a″ and 102 b″ from FIG. 1C (or alternatively 102 a and 102 b from FIG. 1A, or 102 a′ and 102 b′ from FIG. 1B). Additionally, the switched differential amplifier includes a differential pair of transistors Q1 and Q2, a switch SW3 and a current source I. The outputs of the switched input stages are provided to the bases of transistors Q1 and Q2, respectively. The switch S3 selectively connects the emitters of transistors Q1 and Q2 to the current source I. The collectors of transistors Q1 and Q2 form the outputs (Out_(X) and Out_(X) _(—) bar) of the switched differential amplifier 202.

When the switched differential amplifier 202 is “selected,” both input stages 102 a and 102 b are “selected,” and the switch SW3 is switched such that the emitters of transistors Q1 and Q2 are connected to the current source I, as shown in FIG. 2. This will cause the input signals (Vin_(X) and Vin_(X) _(—) bar) to pass in voltage mode, substantially unmodified through the switched input stages 102 a″ and 102 b″, causing the output signals (Iout_(X) and Iout_(X) _(—) bar) to be current mode equivalents of the inputs signals (Vin_(X) and Vin_(X) _(—) bar). With the emitters of transistors Q1 and Q2 connected to the current source I, the transistors Q1 and Q2 are turned on, and the differential pair of transistors Q1 and Q2 act as a normal differential amplifier with the output (Out_(X) and Out_(X) _(—) bar) of the differential amplifier being presented at the collectors of the transistors.

When the switched differential amplifier 202 is “deselected,” both switched input stages 102 a″ and 102 b″ are “deselected” and held at a substantially constant, substantially equal, and below common mode voltage (in accordance with the explanation of FIGS. 1A-1C), and the switch S3 is switched such that the emitters of transistors Q1 and Q2 are not connected to the current source I, input signals (Vin_(X) and Vin_(X) _(—) bar) will be rejected, causing the output signals (Iout_(X) and Iout_(X) _(—) bar) to be substantially reduced. With the emitters of transistors Q1 and Q2 not connected to the current source I, the transistors Q1 and Q2 are turned off. This results in further input signal rejection with substantially zero level current outputs (Out_(X) and Out_(X) _(—) bar) at the collectors of the transistors Q1 and Q2. In this manner, substantially none of the input signals (Vin_(X) and Vin_(X) _(—) bar) will propagate to the outputs (Out_(X) and Out_(X) _(—) bar) of the switched differential amplifier 202, when the amplifier 202 is “deselected”. Thus, when “deselected,” the switched input stages and the differential amplifier provide for double isolation of the input signal.

FIG. 3 shows multiple switched differential amplifiers 202 of FIG. 2 connected as a multiplexer 302. A selector 304 (e.g., a multi-bit decoder) accepts a select signal (e.g., binary data) and turns on exactly one of its outputs based on the select signal. For a more specific example, the selector 304 can have m binary inputs, useful for selecting 1 of 2^(m) outputs. In this manner, the selector 304 is used to “select” one of the switched differential amplifiers 202 ₁-202 _(n) (where n≧2). The selected differential amplifier 202 sets its internal switches to allow the selected analog input to propagate through the differential amplifier. The outputs of the selected switched differential amplifier 202 are connected together (e.g., wired together) with the corresponding outputs of the other switched differential amplifiers, which are each “deselected”. As explained in detail above, the outputs of the “deselected” switched differential amplifier(s) 202 will be substantially equal to zero, and thus, generally not affect the outputs of the multiplexer 302 (Out_(mux) and Out_(mux) _(—) bar).

FIG. 4 shows a multiplexing amplifier 402, according to an embodiment of the present invention. As can be appreciated from FIG. 4, the multiplexing amplifier 402 builds upon the circuits discussed above. More specifically, the multiplexing amplifier 402 is shown as including the multiplexer 302 of FIG. 3, a current mirror CM2, and an output amplifier 404. The current mirror CM2 is used to combine the two current mode output signals of the multiplexer 302 at a gain node N_(G) to produce a single-ended voltage signal, which is passed through the output amplifier 404. The output of the amplifier 404 is fed back as shown, providing for an operational feedback circuit. The difference in the currents at Out_(mux) and Out_(mux) _(—) bar causes the voltage at the gain node N_(G) to move around. The voltage at the gain node N_(G) is fed back to the input of the selected switched differential amplifier 202, such that a final value of output voltage (Vout) of the multiplexing amplifier is reached.

The output amplifier 402 can be a gain amplifier or a simple buffer. In another embodiment, the output amplifier 402 is removed and replaced with a wire, causing the output voltage (Vout) to be the voltage at the gain node N_(G).

One of ordinary skill in the art would appreciate that the above circuits could essentially be flipped by replacing the NPN transistors with PNP transistors, and appropriately adjusting the supply voltage. One of ordinary skill in the art will also appreciate that other types of transistors, such as, but not limited to complimentary-metal-oxide-semiconductor (CMOS) type transistors (i.e., NMOS and PMOS) or junction field effect transistors (JFETs), can alternatively be used. Additionally, one of ordinary skill in the art would understand that fully complimentary versions of the above described circuits are also within the spirit and scope of the present invention.

Multiplexers in accordance with embodiments of the present invention can be used, e.g., to provide video muliplexers. In one specific example, embodiments of the present invention can multiplex RGB signals received from several video sources (e.g., DVD players, VCRs, tuners, digital cameras, etc.) into a single video display (also known as a monitor).

For example, as shown in FIG. 5, embodiments of the present invention can be used to produce a triple 2:1 video multiplexer 502. Such a multiplexer 502 is useful for various types of applications, such as, but not limited to, set-top boxes, in-car navigation/entertainment, servers, security systems, video projectors, notebook computers, broadcast video and video crosspoint switching.

More specifically, the triple 2:1 video multiplexer 502 in FIG. 5 is made up of three 2:1 multiplexers 402 a, 402 b and 402 c, which are each similar to the multiplexer 402 described above with reference to FIG. 4. A first video source 504 a outputs a first RGB video signal, with the R portion of the signal being provided to the 2:1 mux 402 a, the G portion of the signal being provided to the 2:1 mux 402 b, and the B portion of the signal being provided to the 2:1 mux 402 c. A second video source 504 b outputs a second RBG video signal, with the R portion of the signal being provided to the 2:1 mux 402 a, the G portion of the signal being provided to the 2:1 mux 402 b, and the B portion of the signal being provided to the 2:1 mux 402 c. Each 2:1 mux 402 a, 402 b, and 402 c is substantially similar, and thus the details are only shown of the 2:1 mux 402 a.

A source select signal is provided to the selector 304. Based on the source select signal, the selector 304 selects one of two switched differential amplifiers 202 (i.e., 202, or 2022) within each 2:1 mux 402 a, 402 b and 402 c. In this manner, the RGB signal from either the first video source 504 a or the RGB signal from-the second video source 504 b (which may or may not be amplified) is output from the triple 2:1 mux 502 and provided to the display 506 (e.g., a television or other type of monitor).

This is just one example of how the embodiments of the present invention can be used. One of ordinary skill in the art will appreciate that there are numerous other related and unrelated uses for the multiplexer embodiments of the present invention.

The forgoing description is of the preferred embodiments of the present invention. These embodiments have been provided for the purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to a practitioner skilled in the art. Embodiments were chosen and described in order to best describe the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention. Slight modifications and variations are believed to be within the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

1. A multiplexer circuit, comprising a plurality of switched differential amplifier circuits each including a pair of differential inputs and a pair of differential outputs, with each pair of differential inputs accepting a corresponding pair of voltage input signals including a first input signal and a second input signal, wherein each switched differential amplifier circuit comprises: a first switched input stage including an input to accept the first input signal and an output, said first switched input stage configured to, when selected, present a substantially unmodified version of the first input signal at its said output, and said first switched input stage configured to, when deselected, produce a first rejection voltage signal of substantially equal magnitude and opposite polarity to the first input signal in order to reject said first input signal and thereby present a substantially constant voltage at its output regardless of variations in the first input signal; and a second switched input stage including an input to accept the second input signal and an output, said second switched input stage configured to, when selected, present a substantially unmodified version of the second input signal at its said output, and said second switched input stage configured to, when deselected, produce a second rejection voltage signal of substantially equal magnitude and opposite polarity to the second input signal in order to reject said second input signal, and thereby present a substantially constant voltage at its said output regardless of variations in the second input signal.
 2. The circuit of claim 1, wherein only one of said plurality of switched differential amplifiers is selected at a time.
 3. The multiplexer circuit of claim 1, wherein: each said first switched input stage comprises a first transconductance circuit that converts said first input signal to a proportional first current signal that, when said first switched input stage is selected, flows through a first passive resistance in order to produce said first rejection voltage signal; and each said second switched input stage comprises a second transconductance circuit that converts said second input signal to a proportional second current signal that, when said second switched input stage is selected, flows through a second passive resistance in order to produce said second rejection voltage signal.
 4. The multiplexer circuit of claim 1, wherein each said switched input stage comprises: a switched input stage input; a switched input stage output; a resistor having a first terminal and a second terminal, said first terminal of said resistor connected to said switched input stage input, said second terminal of said resistor connected to said switched input stage output; a capacitor connected in parallel with said resistor; and a transconductor having an input and an output, said transconductor input being connected to said first terminal of said resistor, said transconductor output switchably connectable to said switched input stage output; wherein said transconductor output is not connected to said first switched input stage output when said switched input stage is selected; and wherein said transconductor output is connected to said switched input stage output when said switched input stage is deselected.
 5. The circuit of claim 4, wherein each said switched input stage further comprises buffer connected between said switched input stage input and said first terminal of said resistor.
 6. The circuit of claim 4, wherein said transconductor comprises: a current mirror including an input and an output; a further resistor including a first terminal and a second terminal, said first terminal of said further resistor connected to said first terminal of said resistor, said second terminal of said further resistor connected to said current mirror input; and a further capacitor connected in parallel with said further resistor; wherein said current mirror output forms said output of said transconductor.
 7. The circuit of claim 1, wherein each said pair of differential outputs of each said switched differential amplifier circuit includes a first output and a second output, and wherein each said switched differential amplifier circuit further comprises: a first transistor including a base connected to said output of said first switched input stage, a collector forming said first output of said switched differential amplifier circuit, and an emitter; a second transistor including a base connected to said output of said second switched input stage, a collector forming said second output of said switched differential amplifier circuit, and an emitter connected to said emitter of said first transistor; and a current source selectively connectable to said emitters of said first and second transistors.
 8. The circuit of claim 7, wherein: substantially unmodified versions of the first and second input signals are presented at said first and second outputs of said switched differential amplifier circuit when said first and second switched input stages are selected and said current source is connected to said emitters of said first and second transistors.
 9. The circuit of claim 8, wherein: substantially zero level output signals are presented at said first and second outputs of said switched differential amplifier circuit when said first and second switched input stages are deselected and said current source is not connected to said emitters of said first and second transistors.
 10. The circuit of claim 1, wherein each said switched differential amplifier circuit is configured to present a substantially unmodified version of its input signals at its differential outputs when said switched differential amplifier circuit is selected, and to present substantially zero level output signals at its differential outputs when said switched differential amplifier circuit is deselected.
 11. The circuit of claim 1, further comprising: a current mirror including an input and an output; wherein only one of said plurality of switched differential amplifiers is selected at a time; wherein each said pair of differential outputs includes a first output and a second output, and wherein said first outputs are connected together to form a first multiplexer output, and said second outputs are connected together to form a second multiplexer output; and wherein said current mirror input is connected to said first multiplexer output, and said current mirror output is connected to said second multiplexer output.
 12. The circuit of claim 11, further comprising a selector to select one of said plurality of switched differential amplifiers.
 13. The circuit of claim 12, wherein said selector accepts a select signal and selects one of said plurality of switched differential amplifier circuits based on said select signal.
 14. The circuit of claim 13, wherein said selector is a decoder, and wherein said select signal is a binary signal.
 15. The circuit of claim 11, wherein, at said current mirror output, a signal output from said first multiplexer output is combined with a signal output from second multiplexer output to thereby produce a single-ended multiplexer output signal.
 16. The circuit of claim 15, wherein each said pair of differential inputs includes a first input and a second input, wherein said second inputs are connected together, and wherein said connected together second inputs accept a fed back version of said single-ended multiplexer output signal.
 17. The circuit of claim 16, further comprising: an amplifier including an input and an output, said amplifier input accepting said single-ended multiplexer output signal, said amplifier output connected to said connected together second inputs.
 18. The circuit of claim 16, further comprising: a buffer including an input and an output, said buffer input accepting said single-ended multiplexer output signal, said buffer output connected to said connected together second inputs.
 19. The circuit of claim 16, wherein said connected together second inputs are connected to said current mirror output.
 20. A switched input stage circuit for use in a multiplexer, comprising: an input to accept an input signal; and an output; said switched input stage configured to, when selected, present a substantially unmodified version of the input signal at its said output; and said switched input stage configured to, when deselected, produce a rejection voltage signal of substantially equal magnitude and opposite polarity to the input signal in order to reject the input signal and thereby present a substantially constant voltage at its said output regardless of variations in the input signal.
 21. The circuit claim 20, including a transconductance circuit that converts the input signal to a proportional current signal that, when said switched input stage is selected, flows through a passive resistance in order to produce said rejection voltage signal.
 22. The circuit claim 20, including: a resistor having a first terminal and a second terminal, said first terminal of said resistor connected to said switched input stage input, said second terminal of said resistor connected to said switched input stage output; a capacitor connected in parallel with said resistor; and a transconductor having an input and an output, said transconductor input being connected to said first terminal of said resistor, said transconductor output switchably connectable to said switched input stage output; wherein said transconductor output is not connected to said first switched input stage output when said switched input stage is selected; and wherein said transconductor output is connected to said switched input stage output when said switched input stage is deselected.
 23. The circuit of claim 22, wherein said transconductor comprises: a current mirror including an input and an output; a further resistor including a first terminal and a second terminal, said first terminal of said further resistor connected to said first terminal of said resistor, said second terminal of said further resistor connected to said current mirror input; and a further capacitor connected in parallel with said further resistor; wherein said current mirror output forms said output of said transconductor.
 24. A multiplexer circuit, comprising: a plurality of switched differential amplifier circuits each including a pair of differential inputs and a pair of differential outputs, each said switched differential amplifier circuit being configured to present a substantially unmodified version of its input signals at its differential outputs when said switched differential amplifier circuit is selected, and to present substantially zero level output signals at its differential outputs when said switched differential amplifier circuit is deselected, regardless of variations in its input signals; a selector that accepts a select signal and selects one of said plurality of switched differential amplifier circuits based on said select signal; and a current mirror including an input and an output; wherein each said pair of differential outputs includes a first output and a second output, and wherein said first outputs are connected together to form a first multiplexer output, and said second outputs are connected together to form a second multiplexer output; wherein said current mirror input is connected to said first multiplexer output, and said current mirror output is connected to said second multiplexer output, such that at said current mirror output, a signal output from said first multiplexer output is combined with a signal output from second multiplexer output to thereby produce a single-ended multiplexer output signal; and wherein each said pair of differential inputs includes a first input and a second input, wherein said second inputs are connected together, and wherein said connected together second inputs accept a fed back version of said single-ended multiplexer output signal.
 25. A multiplexer circuit, comprising: a plurality of switched differential amplifier circuits each including a pair of differential inputs and a pair of differential outputs, with each said pair of differential inputs accepting a corresponding pair of input signals, and each said pair of differential outputs including a first output and a second output at which are output corresponding first and second output signals; wherein only one of said plurality of switched differential amplifiers is selected at a time, and wherein each switched differential amplifier circuit comprises a first switched input stage including an input to accept the first input signal and an output, said first switched input stage configured to present a substantially unmodified version of the first input signal at its output when said first switched input stage is selected, and to present a substantially constant voltage at its output when said first switched input stage is deselected, regardless of variations in the first input signal; a second switched input stage including an input to accept the second input signal and an output, said second switched input stage configured to present a substantially unmodified version of the second input signal at its output when said second switched input stage is selected, and to present a substantially constant voltage at its output when said second switched input stage is deselected, regardless of variations in the second input signal; a first transistor including a base connected to said output of said first switched input stage, a collector forming said first output of said switched differential amplifier circuit, and an emitter; a second transistor including a base connected to said output of said second switched input stage, a collector forming said second output of said switched differential amplifier circuit, and an emitter connected to said emitter of said first transistor; and a current source selectively connectable to said emitters of said first and second transistors.
 26. The circuit of claim 25, wherein each said switched input stage comprises: a switched input stage input; a switched input stage output; a resistor having a first terminal and a second terminal, said first terminal of said resistor connected to said switched input stage input, said second terminal of said resistor connected to said switched input stage output; a capacitor connected in parallel with said resistor; and a transconductor having an input and an output, said transconductor input being connected to said first terminal of said resistor, said transconductor output switchably connectable to said switched input stage output; wherein said transconductor output is not connected to said first switched input stage output when said switched input stage is selected; and wherein said transconductor output is connected to said switched input stage output when said switched input stage is deselected.
 27. The circuit of claim 26, wherein each said switched input stage further comprises buffer connected between said switched input stage input and said first terminal of said resistor.
 28. The circuit of claim 26, wherein said transconductor comprises: a current mirror including an input and an output; a further resistor including a first terminal and a second terminal, said first terminal of said further resistor connected to said first terminal of said resistor, said second terminal of said further resistor connected to said current mirror input; and a further capacitor connected in parallel with said further resistor; wherein said current mirror output forms said output of said transconductor.
 29. A multiplexer circuit, comprising: a plurality of switched differential amplifier circuits each including a pair of differential inputs and a pair of differential outputs, with each pair of differential inputs accepting a corresponding pair of input signals including a first input signal and a second input signal; wherein each switched differential amplifier includes a first switched input stage and a second switched input stage, with each said switched input stage comprising a switched input stage input; a switched input stage output; a resistor having a first terminal and a second terminal, said first terminal of said resistor connected to said switched input stage input, said second terminal of said resistor connected to said switched input stage output; a capacitor connected in parallel with said resistor; and a transconductor having an input and an output, said transconductor input being connected to said first terminal of said resistor, said transconductor output switchably connectable to said switched input stage output; wherein said transconductor output is not connected to said first switched input stage output when said switched input stage is selected; and wherein said transconductor output is connected to said switched input stage output when said switched input stage is deselected.
 30. The circuit of claim 29, wherein only one of said plurality of switched differential amplifiers is selected at a time.
 31. The circuit of claim 29, wherein each said switched input stage further comprises buffer connected between said switched input stage input and said first terminal of said resistor.
 32. The circuit of claim 29, wherein said transconductor comprises: a current mirror including an input and an output; a further resistor including a first terminal and a second terminal, said first terminal of said further resistor connected to said first terminal of said resistor, said second terminal of said further resistor connected to said current mirror input; and a further capacitor connected in parallel with said further resistor; wherein said current mirror output forms said output of said transconductor.
 33. The circuit of claim 29, wherein each said pair of differential outputs of each said switched differential amplifier circuit includes a first output and a second output, and wherein each said switched differential amplifier circuit further comprises: a first transistor including a base connected to said output of said first switched input stage, a collector forming said first output of said switched differential amplifier circuit, and an emitter; a second transistor including a base connected to said output of said second switched input stage, a collector forming said second output of said switched differential amplifier circuit, and an emitter connected to said emitter of said first transistor; and a current source selectively connectable to said emitters of said first and second transistors.
 34. The circuit of claim 29, further comprising: a current mirror including an input and an output; wherein only one of said plurality of switched differential amplifiers is selected at a time; wherein each said pair of differential outputs includes a first output and a second output, and wherein said first outputs are connected together to form a first multiplexer output, and said second outputs are connected together to form a second multiplexer output; and wherein said current mirror input is connected to said first multiplexer output, and said current mirror output is connected to said second multiplexer output.
 35. The circuit of claim 34, further comprising a selector to select one of said plurality of switched differential amplifiers.
 36. The circuit of claim 35, wherein, at said current mirror output, a signal output from said first multiplexer output is combined with a signal output from second multiplexer output to thereby produce a single-ended multiplexer output signal.
 37. The circuit of claim 36, wherein each said pair of differential inputs includes a first input and a second input, wherein said second inputs are connected together, and wherein said connected together second inputs accept a fed back version of said single-ended multiplexer output signal.
 38. The circuit of claim 37, further comprising: an amplifier including an input and an output, said amplifier input accepting said single-ended multiplexer output signal, said amplifier output connected to said connected together second inputs.
 39. The circuit of claim 37, further comprising: a buffer including an input and an output, said buffer input accepting said single-ended multiplexer output signal, said buffer output connected to said connected together second inputs.
 40. The circuit of claim 37, wherein said connected together second inputs are connected to said current mirror output.
 41. A triple 2:1 multiplexer system, comprising: a first 2:1 multiplexer that receives an R portion of a first RGB signal from a first video source and an R portion of a second RGB signal from a second video source, a second 2:1 multiplexer that receives a G portion of the first RGB signal from the first video source and a G portion of the second RGB signal from the second video source, a third 2:1 multiplexer that receives a B portion of the first RGB signal from the first video source and a B portion of the second RGB signal from the second video source, wherein each of the first, second and third 2:1 multiplexers includes a pair of switched differential amplifier circuits; and a selector that accepts a select signal and selects one of said pair of switched differential amplifier circuits within each of said first, second and third 2:1 multiplexers, based on said select signal, to thereby provide either the first RGB signal or the second RGB signal to a display; wherein each switched differential amplifier circuit includes a first switched input stage including an input to accept a first input signal and an output, said first switched input stage configured to present a substantially unmodified version of the first input signal at its output when said first switched input stage is selected, and to present a substantially constant voltage at its output when said first switched input stage is deselected, regardless of variations in the first input signal; and a second switched input stage including an input to accept a second input signal and an output, said second switched input stage configured to present a substantially unmodified version of the second input signal at its output when said second switched input stage is selected, and to present a substantially constant voltage at its output when said second switched input stage is deselected, regardless of variations in the second input signal; and wherein each said switched input stage includes a switched input stage input; a switched input stage output; a resistor having a first terminal and a second terminal, said first terminal of said resistor connected to said switched input stage input, said second terminal of said resistor connected to said switched input stage output; a capacitor connected in parallel with said resistor; and a transconductor having an input and an output, said transconductor input being connected to said first terminal of said resistor, said transconductor output switchably connectable to said switched input stage output; wherein said transconductor output is not connected to said first switched input stage output when said switched input stage is selected; and wherein said transconductor output is connected to said switched input stage output when said switched input stage is deselected.
 42. The multiplexer circuit of claim 1, wherein each said switched input stage comprises: a switched input stage input; a switched input stage output; a resistor having a first terminal and a second terminal, said first terminal of said resistor connected to said switched input stage input, said second terminal of said resistor connected to said switched input stage output; a capacitor connected in parallel with said resistor; and a first transconductor having an input and an output, said first transconductor input being connected to said first terminal of said resistor, said first transconductor output switchably connectable to said switched input stage output; a second transconductor having an input and an output, said second transconductor input being connected to said first terminal of said resistor, said second transconductor output switchably connectable to said first terminal of said resistor; wherein when said switched input stage is selected, said first transconductor output is not connected to said first switched input stage output, and said second transconductor output is not connected to said first terminal of said resistor; and wherein when said switched input stage is deselected, said first transconductor output is connected to said switched input stage output, and said second transconductor output is connected to said first terminal of said resistor.
 43. The multiplexer circuit of claim 29, further comprising: a further transconductor (TC2) having an input and an output, said further transconductor input being connected to said first terminal of said resistor, said further transconductor output switchably connectable to said first terminal of said resistor; wherein said further transconductor output is not connected to said first terminal of said resistor when said switched input stage is selected; and wherein said further transconductor output is connected to said first terminal of said resistor when said switched input stage is deselected. 